Physiological Importance of Pectin Modifying Genes During Rice Pollen Development.

Yu-Jin Kim,Seung-Yeon Kang,Ki-Hong Jung,Eui-Jung Kim,Ho Young Jeong,Chanhui Lee,Jeniffer Silva,Soon Ki Park

doi:10.3390/ijms21144840

Abstract

Although cell wall dynamics, particularly modification of homogalacturonan (HGA, a major component of pectin) during pollen tube growth, have been extensively studied in dicot plants, little is known about how modification of the pollen tube cell wall regulates growth in monocot plants. In this study, we assessed the role of HGA modification during elongation of the rice pollen tube by adding a pectin methylesterase (PME) enzyme or a PME-inhibiting catechin extract (Polyphenon 60) to in vitro germination medium. Both treatments led to a severe decrease in the pollen germination rate and elongation. Furthermore, using monoclonal antibodies toward methyl-esterified and de-esterified HGA epitopes, it was found that exogenous treatment of PME and Polyphenon 60 resulted in the disruption of the distribution patterns of low- and high-methylesterified pectins upon pollen germination and during pollen tube elongation. Eleven PMEs and 13 PME inhibitors (PMEIs) were identified by publicly available transcriptome datasets and their specific expression was validated by qRT-PCR. Enzyme activity assays and subcellular localization using a heterologous expression system in tobacco leaves demonstrated that some of the pollen-specific PMEs and PMEIs possessed distinct enzymatic activities and targeted either the cell wall or other compartments. Taken together, our findings are the first line of evidence showing the essentiality of HGA methyl-esterification status during the germination and elongation of pollen tubes in rice, which is primarily governed by the fine-tuning of PME and PMEI activities.

Highlights

All plant cells are encased by walls with a distinct polysaccharide composition
50% of the pollen grains germinated at a concentration of 1 units·mL−1 pectin methylesterase (PME), nearly 2% germinated at a concentration of 3 units·mL−1 PME, and no pollen germination was observed at higher concentrations of 4–15 unit·mL−1 PME (Figure 1N)
Our findings suggest that the plasticity and integrity of cell walls during pollen tube growth are primarily governed by the fine-tuning of PME and PME inhibitors (PMEIs) activities, which is one of the critical factors for pollen germination and elongation

Summary

Introduction

All plant cells are encased by walls with a distinct polysaccharide composition. Some specialized cells, including those of root hairs and pollen tubes, undergo rapid directional elongation [1,2,3]. Once differentiated, these types of cells depend upon the coordinated action of cell wall-modifying enzymes (CWMEs), which promote wall loosening and extensibility by disrupting chemical bonds or selectively removing specific polysaccharide side chains [4]. Compositional analysis of grass cell walls has revealed that pectin accounts for approximately 5% of the cell wall components [8,9]

Methods

Results

Conclusion